1. Field of the Invention
The present invention relates to telecommunication optical cables and in particular it relates to an optical telecommunication cable having a controlled fiber length. The present invention further relates to a method and an apparatus for manufacturing such a controlled fiber length optical cable.
2. Description of the Related Art
Several different telecommunication optical cables are known in the art. A widely used type of optical cable is the so-called Multi Loose Tube (MLT) cable, an example of which is described in U.S. Pat. No. 5,999,677. A typical MLT cable comprises: a central strength member; a number of tubes containing loosely placed optical fibers; a mechanical reinforcing layer, for example a thread made of glass or of an aramid material arranged around the tubes; and a protective outer sheath. The tubes containing the optical fibers are typically stranded around the central tube according to a unidirectional helix or a bidirectional helix.
In the description that follows, use will be made indistinctly of the terms “bidirectional helix”, “open helix” or “SZ helix” to signify a trajectory along a cylindrical surface resulting from the combination of the translatory movement in a direction parallel to a central axis with an alternate rotary movement about the axis itself. Basically, this type of trajectory is different from a “unidirectional” or “closed” helix trajectory since the winding around the central axis is not always performed in the same direction, but alternately in a clockwise direction and anti-clockwise direction.
As said above, in a known MLT cable, a number of optical fibers are arranged within a tube in a loose manner, namely the fibers are not tightly constrained within the tubes.
The central rod operates as an antibuckling element. It is characterized by a low thermal expansion coefficient and is made of a material having a rather low modulus of elasticity. The yarns of aramid material or glass form reinforcement against traction forces but they provide no substantial resistance to axial compression forces applied to the cable.
Generally, both the external sheath and the tubes are made of a plastic material and give a low contribution to the traction resistance of the whole optical cable.
An important technical requirement for an optical cable is that it should be able to resist to lay pulling, namely the traction force applied during the step of laying the cable, thus limiting its elongation. In addition to such a characteristic, the length of an optical cable should not be affected by thermal changes, due for instance to environmental temperatures changes (night, day, winter, summer, . . . ), so that the fibers contained therein do not become stressed. In other words, it is highly desirable that optical cable contractions due to temperature changes are particularly reduced.
Typically, a MLT cable, as soon as it becomes subjected to a temperature reduction, tends to contract. It is known that the optical fibers do not experience the same contraction and tend to become bent. In fact, as a consequence of the cable contraction, the optical fibers may come into contact with the inner wall of the tube where they are placed and form fundamentally random bendings.
The generated fiber optic bendings could be divided into microbendings and macrobendings but, in any case, they originate unwanted optical attenuation increases.
In addition to the above problems of the known Multi Loose Tube optical cables, a further disadvantage of such an optical cable category lies in the size. In other words, mainly because of the presence of the central rod, and due to the fact that the fibers are loose within the tubes, the diameter of such MLT optical cables is rather high. A high diameter optical cable is undesirable both for installation and production reasons (high costs, disadvantageous installation, bulky winding bobbin).
A further disadvantage of the known Multi Loose Tube optical cables is that the tube are made of a material which is rather tough such as, for instance, high density polythene or polybutylene therephtalate. This results in the need to use a special tool for removing or stripping the tubes. In other words, a technical person desiring to strip the tube should use a tool and can not operate by using his hands only.
In view of the above inconveniences, with the aim to solve the above problems of the known MLT optical cables, it has been designed a new type of telecommunication optical cable, known as “microbundle optical cable”.
A microbundle optical cable is known, for instance, from WO00/58768 and comprises a number of microbundles, an inner tube circling the microbundles and an external sheath (made, for example, of polyethylene or the like) covering the inner tube.
Two or more reinforcing rods, for example made of glass reinforced plastic, are typically arranged in the external sheath. The external sheath may further comprise ripcords. Optionally, a wrapping tape, made of paper for instance, may be placed between the external sheath inner wall and the inner tube external wall for forming a thermal barrier and for decoupling purposes.
In turn, each microbundle of the microbundle optical cable typically comprises a microsheath and a number of optical fibers arranged parallel to each other within the microsheath.
With respect to the tubes used in the MLT optical cables previously described, the microsheaths of the microbundle optical cables are typically smaller and thinner. For example, in a typical MLT optical cable, a tube containing twelve optical fibers has an inner diameter comprised-between about 1.6 mm and 1.8 mm, an external diameter comprised between about 2.2 mm and 2.8 mm and a thickness comprised between 0.3 mm and 0.5 mm, while in a typical microbundle optical cable a microsheath containing the same number of optical fibers (twelve) has an inner diameter of about 1.1 mm, an external diameter of about 1.4 mm and a thickness of about 0.15 mm.
The microsheath is typically made of a material having a reduced modulus of elasticity and a low ultimate elongation, such as PVC. Advantageously, the use of the above material for forming a thin microsheath also results in a microsheath that is easier to remove or to strip, just using fingers or fingernails.
In the optical cable according to WO/58768, the optical fibers are rectilinearly arranged inside the microsheath, while the microbundles may be rectilinearly arranged or S-Z stranded.
A microbundle optical cable fundamentally is advantageous with respect to a MLT optical cable, as said above, because its size and weight are considerably reduced.
Notwithstanding the above advantages, it has been found that a known microbundle optical cable still has considerable and highly undesirable optical attenuation problems.
Several tests and experimentations have been conducted by the Applicant with the aim to discover the reason of such optical attenuation problems. The Applicant has observed that, whenever one of the microbundles or the inner tube transits on a cylindrical surface, such as during the winding or unwinding step, a not negligible movement of the optical fibers or the microbundles was taking place. In particular, some of the optical fibers or microbundles remain straight, while some others tend to ripple.
According to the Applicant, this is mainly due to the fact that the optical fibers lay on different positions within a microbundle. During manufacturing, the just formed microbundle is wound on a capstan pulley that provides the required traction force. When the microbundle leaves the pulley, the optical fibers that were positioned closer to the pulley's center result to be shorter than the optical fibers that were positioned more peripherally. The length difference can be of the order of one or more ‰. Thus the fibers don't have an equalized length. This fact results in that, at the exit of the capstan, there are straight fibers and rippled fibers.
A similar problem occurs when the inner tube is manufactured starting from the microbundles, if the microbundles are provided straight (i.e. parallel to each other). Again, passage of the inner tube on the circular surface of a capstan results in a different length of the microbundles within the inner tube, which different length is related to the different distance of the microbundles from the center of the capstan pulley. The groups of fibers contained in the different microbundles will show a corresponding length difference.
Thus, the Applicant has realized that the relatively high optical attenuation of a typical microbundle optical cable is caused by a too high variation of the optical fiber length.
The Applicant has verified that, for a microbundle optical cable comprising twelve microbundles, each containing twelve optical fibers, microbundles with a difference of fiber length in the average of about 0.3% and inner tubes with a difference of microbundle length in the average of 0.4% are produced.
These length differences result in microbending and/or macrobending problems that will negatively affect the cable attenuation performances.
The Applicant observes that the problems previously addressed, in particular that of a length difference between the fibers contained in a same microbundle, is characteristic of loose optical cables which, as previously stated, differ from tight cables in that the fibers are loosely contained in the surrounding sheath.
A tight cable is for example described in U.S. Pat. No. 5,155,789. The disclosed cable comprises a series of optical fibers, the fibers being split into modules each of which is enveloped by a thin supporting sheath that easily torn, the sheaths being in contact with the optical fibers, and a protective covering being in contact with the supporting sheath. The fibers within a supporting sheath and the modules within the covering may be assembled without twisting, or with twisting at a continuous pitch or at an alternating pitch.
According to the Applicant, when a tight cable as described in U.S. Pat. No. 5,155,789 is wound on a capstan, the outer optical fibers, due to the friction between the optical fibers and the sheath, tend to resiliently stretch. When the sheath becomes straight again, after leaving the capstan, the optical fibers return in their original state. It is known that typical optical fibers are able to elastically resist to the elongations experienced during winding on the capstan. In other words, during the manufacturing of a tight cable, the optical fibers thereof do not experience, according to the Applicant, any permanent length difference.